Block 12 · Post-MCAS · Final exam prep

Codon chart drill

The final exam tests translationThe second step of protein synthesis: mRNA is read at the ribosome and used to build a protein. in detail. This block is a heavy drill on reading the codon chart, translating longer mRNA chains, and running the full DNA-to-protein pipeline. If you can do these problems without hesitation, you are ready.

What you need to know cold

central-dogmaThe flow of genetic information in a cell: DNA → RNA → protein.: DNA → mRNA → protein. This is always the order.
transcriptionThe first step of protein synthesis: a DNA gene is copied into a strand of mRNA. copies DNA into mrnaMessenger RNA — a single-stranded copy of a gene that carries the message from DNA to the ribosome. in the nucleus. The one change: T becomes U.
translationThe second step of protein synthesis: mRNA is read at the ribosome and used to build a protein. reads mRNA at the ribosomeThe cell structure where proteins are built. The ribosome reads mRNA and links amino acids together. to build a protein from An amino acid is a small molecule that is the **building block of a protein**. Proteins are long chains of amino acids linked together. There are about 20 different amino acids that living things use. Each codon on an mRNA strand codes for one amino acid.s.
A codonA group of three mRNA bases that codes for one amino acid. is 3 mRNA bases. Each codon = 1 amino acid. Bases ÷ 3 = number of codons.
Start codon: AUG = methionine. Translation begins here.
Stop codons: UAA, UAG, UGA = no amino acid. Translation ends here.
You will have a codon chart on the test. You do not memorize codons — you look them up.
The final can give you a DNA strand and ask for the protein. That means: transcribe first (DNA → mRNA), then translate (mRNA → protein).

The Big Rule for this block

3 bases = 1 codon = 1 amino acid. Split from the left. Stop means stop.

Every translation problem comes down to the same three moves: split the mRNA into groups of 3 starting from the left, look each codon up on the chart, and stop when you hit UAA, UAG, or UGA.

Key vocabulary in 8 languages

Words from this block. Use the row in your home language to help your memory. Many of these words are similar across languages because they come from Greek and Latin roots.

English	Español	Português	Français	Italiano	Kreyòl	Tiếng Việt	العربية
DNA	ADN	DNA / ADN	ADN	DNA	ADN	ADN / DNA	DNA / الدنا(dī-en-ey / ad-dinā)
RNA / mRNA	ARN	RNA / ARN	ARN	RNA	ARN	ARN / RNA	RNA / الرنا(ar-RNA / ar-rinā)
codon	codón	códon	codon	codone	kodon	côđon / bộ ba mã hóa	كودون / رامزة(kūdūn / rāmiza)
transcription	transcripción	transcrição	transcription	trascrizione	transkripsyon	phiên mã	نسخ(naskh)
translation	traducción	tradução	traduction	traduzione	tradiksyon	dịch mã	ترجمة(tarjama)
amino acid	aminoácido	aminoácido	acide aminé	amminoacido	asid amine	axit amin	حمض أميني(ḥamḍ amīnī)
ribosome	ribosoma	ribossomo	ribosome	ribosoma	ribozòm	ribosome / ribôxôm	ريبوسوم(rībūsūm)
protein	proteína	proteína	protéine	proteina	pwoteyin	protein / prô-tê-in	بروتين(brūtīn)

All 8 rows use the verified translations from the Quick Reference vocabulary table. Vietnamese and Arabic translations were verified by GPT-5 and Gemini. Romance-language translations (Spanish, Portuguese, French, Italian, Haitian Kreyòl) rely on cognate consistency. If a term feels unfamiliar to a native speaker, please tell Ms Brandolini.

The full picture

Codon chart drill — mastering translation for the final

What this reading is about

You already learned transcriptionThe first step of protein synthesis: a DNA gene is copied into a strand of mRNA. and translationThe second step of protein synthesis: mRNA is read at the ribosome and used to build a protein. in Block 2. This reading is a drill review for the final exam. The final tests codon chart use in detail — longer chains, DNA-to-protein pipelines, and start/stop codons. This page walks through the mechanics step by step so you can practice until they are automatic.

Quick review: the central dogma

The central-dogmaThe flow of genetic information in a cell: DNA → RNA → protein. is the order information flows in every cell:

DNA → mRNA → protein

Transcription (DNA → mrnaMessenger RNA — a single-stranded copy of a gene that carries the message from DNA to the ribosome.) happens in the nucleus.
Translation (mRNA → protein) happens at the ribosomeThe cell structure where proteins are built. The ribosome reads mRNA and links amino acids together..

The final exam can ask you to start from a DNA strand and end at a protein. That means you need to do both steps in sequence.

Step 1 review: transcription (DNA → mRNA)

To transcribe dnaThe molecule that carries the genetic instructions for life. Shaped like a twisted ladder. into mRNA, pair each DNA base with its RNA complement. The one change from DNA pairing: T becomes U.

DNA A → mRNA U
DNA T → mRNA A
DNA C → mRNA G
DNA G → mRNA C

Worked example. Transcribe the DNA template strand T-A-C-G-C-T-A-A-T-G-G-A-A-C-T into mRNA.

Go letter by letter: T→A, A→U, C→G, G→C, C→G, T→A, A→U, A→U, T→A, G→C, G→C, A→U, A→U, C→G, T→A.
mRNA result: A-U-G-C-G-A-U-U-A-C-C-U-U-G-A.

Step 2 review: translation (mRNA → protein)

The ribosomeThe cell structure where proteins are built. The ribosome reads mRNA and links amino acids together. reads the mRNA three bases at a time. Each group of three is a codonA group of three mRNA bases that codes for one amino acid.. Each codon tells the ribosome which An amino acid is a small molecule that is the **building block of a protein**. Proteins are long chains of amino acids linked together. There are about 20 different amino acids that living things use. Each codon on an mRNA strand codes for one amino acid. to add to the growing protein chain.

How to read the codon chart

On the final exam, you will have a codon chart. Here is the strategy to use it efficiently:

Split the mRNA into groups of 3 — start from the left and mark off every three bases. Example: AUGCGAUUACCUUGA becomes AUG | CGA | UUA | CCU | UGA.
Find each codon on the chart. Most charts are organized by the first base, then the second, then the third. Follow the chart's layout — first base picks the row, second base picks the column, third base narrows to the exact amino acid.
Write down the amino acid. Keep a running list as you go through each codon.
Stop when you hit a stop codon. UAA, UAG, and UGA code for "Stop" — no amino acid. The protein is done.

Start and stop codons

Two special codons you must know for the final:

Start codon: AUG — codes for methionine. This is where translation begins. Almost every protein starts with methionine.
Stop codons: UAA, UAG, UGA — these do NOT code for any amino acid. When the ribosome reaches a stop codon, it releases the finished protein.

On the codon chart, look up AUG and you will see "Met" (methionine). Look up UAA, UAG, or UGA and you will see "Stop."

Codon math

The math rule is simple:

Number of bases ÷ 3 = number of codons

6 bases → 2 codons → up to 2 amino acids
12 bases → 4 codons → up to 4 amino acids
18 bases → 6 codons → up to 6 amino acids
30 bases → 10 codons → up to 10 amino acids

"Up to" because one of those codons might be a stop codon, which adds no amino acid.

Full pipeline worked example: DNA to protein

Worked example. A DNA template strand reads: T-A-C-C-G-A-A-A-T-A-C-T. What is the amino acid sequence of the protein?

Transcribe DNA → mRNA: T→A, A→U, C→G, C→G, G→C, A→U, A→U, A→U, T→A, A→U, C→G, T→A. mRNA = AUG-CGU-AUA-CGA.
Split into codons: AUG | CGU | AUA | CGA (4 codons).
Look up each codon: AUG = Met (start), CGU = Arg, AUA = Ile, CGA = Arg.
Amino acid chain: Met — Arg — Ile — Arg (4 amino acids).

No stop codon appeared in this sequence, so this is a partial gene. On the final, the question will tell you where to start and stop, or there will be a stop codon in the sequence.

Longer example. mRNA sequence: AUG-UUU-GCA-CCG-UAC-UGA. Translate it.

AUG = Met (start).
UUU = Phe.
GCA = Ala.
CCG = Pro.
UAC = Tyr.
UGA = Stop. Translation ends here.

Protein: Met — Phe — Ala — Pro — Tyr (5 amino acids). The stop codon does not add an amino acid.

Final exam traps to watch for

"How many amino acids?" — Don't forget that a stop codon adds NO amino acid. If you have 6 codons and the last one is a stop, the protein has 5 amino acids, not 6.
"Transcribe this DNA" — Remember that RNA uses U, not T. If your answer has a T in it, go back and fix it.
"Where does translation happen?" — The ribosome. Not the nucleus, not the mitochondria.
"What does mRNA do?" — It carries the genetic message from the DNA in the nucleus to the ribosome in the cytoplasm.
DNA has two strands — The question will tell you which strand is the template. If it just gives you one strand, use that one.

Pictures to recognize on the test

The picture shows…	The answer is…
A DNA strand being copied into a strand with U instead of T.	Transcription. DNA → mRNA. The U replacing T is the giveaway.
An mRNA strand split into groups of 3 bases, with amino acids attached below each group.	Translation. Each group of 3 = one codon = one amino acid.
A circular or grid chart with 3-letter codes (AUG, UUU, GCA…) matched to amino acid names.	Codon chart. Use it to look up which amino acid each codon codes for.
A ribosome sliding along an mRNA strand, with tRNA molecules bringing amino acids.	Translation in progress. The ribosome is reading codons and building a protein.
A chain of amino acids linked together.	A protein (polypeptide chain). This is the final product of translation.
The full flow: DNA → mRNA → ribosome → protein.	Central dogma / protein synthesis. The complete pipeline from gene to protein.

Pattern rules

If the question says…	Pick…
"Transcribe this DNA strand."	Pair each base: A↔U, T↔A, C↔G, G↔C. Remember: T becomes U in RNA.
"How many codons in this mRNA?"	Number of bases ÷ 3. (12 bases = 4 codons, 18 bases = 6 codons.)
"Translate this mRNA sequence."	Split into groups of 3, look each up on the codon chart. Start from the left.
"What amino acid does AUG code for?"	Methionine (Met). AUG is the start codon.
"What happens when the ribosome reaches UAA, UAG, or UGA?"	Translation stops. No amino acid is added. The protein is released.
"Where does translation happen?"	At the ribosome. Not the nucleus, not the mitochondria.
"What does mRNA do?"	Carries the genetic message from DNA in the nucleus to the ribosome.
"Given a DNA strand, what protein is produced?"	Two steps: first transcribe (DNA → mRNA), then translate (mRNA → protein using the codon chart).

Where to practice

Practice Block 12 codon chart and translation problems via the 2025 MCAS Biology test on Pear Assessment. Focus on the protein synthesis questions. Try the practice without looking at this page first. If you get stuck, come back, look up the pattern, then try again.